The present invention relates to a doubly-fed motor/generator using an excitation power converter or to a doubly-fed electric machine.
A doubly-fed generator using an excitation power converter, or a doubly-fed variable-speed electric machine is able to control a reactive power output, as does a conventional synchronous generator/motor, and also to realize a high-speed torque control or a high-speed active power control within a rotation speed range around a synchronous speed. This gives the doubly-fed electric machines an advantage of being able to optimally operate prime movers, such as hydraulic power plants and wind power generation systems, in a wider range of conditions than can the conventional synchronous machines. They have another advantage of being able to contribute to stabilizing the frequency of the utility power system or power grid by temporarily releasing or absorbing energy of a rotating flywheel to and from the power grid.
As to the power converter for secondary excitation, not only does it have a far larger capacity than that of the excitation power converter for the conventional synchronous machine, though its capacity can be made smaller than that of an armature of the generator, its circuit is more complex. Therefore an overcurrent capability and an exciter ceiling voltage, equivalent to those of the excitation power converter of the conventional synchronous machine, are both economically difficult to secure.
For this reason, in the event of a fault occurring in a power grid, it is general practice to activate a short-circuit unit to allow an excitation winding overcurrent to bypass the excitation power converter, minimizing the overcurrent capacity. Where a resistor is inserted to reduce the current flowing through the short-circuit unit, in particular, the torque of the doubly-fed electric machine changes rapidly to that of the wound-rotor induction machine shorted with a secondary resistor and also wildly varies depending on the rotation speed. As a result, even if the generator can be managed to remain in operation, the large and rapid changes in the torque cause significant disturbances in the power grid, making it impossible to sustain the continued supply of electricity to consumers.
As one method to deal with this drawback, Hitachi Review, 1995 Vol. 44, describes a procedure whereby, when an excitation overvoltage is detected by a cycloconverter made up of anti-parallelly connected thyristors, a power converter unit of a polarity opposite to an excitation current command is turned on to continue its operation. Another method is also practiced which involves computing a loss from current values of power devices, calculating junction temperatures of the power devices from cooling water temperatures at all times and providing an overcurrent protection with a desired time delay by using the junction temperatures for protection in order to allow for a continued operation until a limit on the power device junction temperature is reached. This minimizes the activation of a short-circuit unit as practically as possible.
As self-turn-off power devices have been making remarkable technological advances in recent years, self-commutated converters are moving toward larger capacity and higher voltage by employing power devices such as IGBTs (Insulated Gate Bipolar Transistors). The self-commutated converters have advantages not found in externally commutated converters, such as capabilities to adjust a converter power factor and optimize the outflow of generated harmonics by adjusting a pulse-width-modulated (PWM) frequency. On the other hand, when used on doubly-fed generators that are required to continue their operation even during abnormalities on the power grid side, the power converters using the self-turn-off power devices have difficulty securing a short-time overcurrent capability economically. This is because, while the conventional power devices, such as thyristors, are able to provide a short-time overcurrent capability up to the junction temperature upper limit, the self-turn-off devices' current rating is determined by an instantaneous current interrupting capacity.